In the recording, Clinton, who endorsed Romanoff in June, praised Romanoff's record as Speaker of the Colorado State House:
"Hi, this is Bill Clinton. I am supporting Andrew Romanoff for the U.S. Senate and I hope that you will vote for him in Tuesday's Democratic primary because he has really good ideas on the economy.
"He has proven himself to be one of the most effective legislative leaders in the entire country, and I think that he has the best chance of holding the seat in November ... And please mark the top line Andrew Romanoff for U.S. Senate. Thank you."
Last week President Barack Obama, who endorsed incumbent Michael Bennet nearly a year ago, held a conference call with voters to rally support.
On the Republican side, candidate Jane Norton had Senator and former presidential candidate John McCain stump for her over the weekend.
The latest polling shows close races in both Senate primaries.